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Network Working Group                                          M. Larsen
Internet-Draft                                                  Ericsson
Expires: November 30, 2004                                     June 2004

                           Port Randomisation

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
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Copyright Notice

   Copyright (C) The Internet Society (2004).


   The Internet protocols TCP and UDP are both vulnerable to data
   injection attacks.  The consequences of injected data range from
   nuisance through broken connections and corrupted local data.

   This document describe a simple, efficient and client local method
   for random selection of the client port number, such that the
   possibility of an attacker guessing the exact value is reduced.  This
   is not a replacement for cryptographic methods such as IPsec or the

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   TCP MD5 signature option.  However, the proposed method provides
   improved security/obfuscation with very little effort and without any
   key management overhead.

   The proposed algorithm has similarities with the algorithm proposed
   in [RFC1948].

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Randomising Ports  . . . . . . . . . . . . . . . . . . . . . .  4
     2.1   Ephemeral Port Range . . . . . . . . . . . . . . . . . . .  4
     2.2   Choosing the Port  . . . . . . . . . . . . . . . . . . . .  4
     2.3   Secret Key . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.4   Choosing Algorithm . . . . . . . . . . . . . . . . . . . .  7
   3.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   4.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   5.1   Normative References . . . . . . . . . . . . . . . . . . . . 11
   5.2   Informative References . . . . . . . . . . . . . . . . . . . 11
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 12
       Intellectual Property and Copyright Statements . . . . . . . . 13

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1.  Introduction

   The Internet protocols TCP [RFC793] and UDP [RFC768] are both
   vulnerable to data injection attacks.  The consequences of injected
   data (which may be both control data and payload data) range from
   nuisance through broken connections and corrupted local data

   To make such attacks possible, the attacker must usually know both
   local and peer IP addresses and ports (the connection four-tuple) and
   any sequence numbers involved in the communication.  Alternatively
   the attacker must make a good prediction of the these parameters to
   reduce the search space.  The connection must also exist long enough
   for the attack to be executed.  Such attacks are feasible as
   illustrated by [Watson].

   Besides IP addresses, Internet protocols like TCP and UDP use a set
   of ports (local and peer) to identify communication endpoints.
   Services are usually located at fixed, 'well-known' ports [IANA] at
   the host supplying the service (the server).  Client applications
   connecting to any such service will contact the server by specifying
   the server IP address and service port number.  The IP address and
   port number of the client are normally left unspecified by the client
   application and thus chosen automatically by the client networking
   stack.  Ports chosen automatically by the networking stack are known
   as ephemeral ports [Stevens].

   While the well-known service port and both server and client IP
   address may be available to an attacker, the ephemeral port of the
   client are usually unknown and must be guessed.

   This document describe a method for random selection of the ephemeral
   port, thereby reducing the possibility of an off-path attacker
   guessing the exact value.  This is not a replacement for
   cryptographic methods such as IPsec or the TCP MD5 signature option
   [RFC2385].  However, the proposed method provides improved
   obfuscation with very little effort and without any key management

   The mechanism is a local modification and may be incrementally
   deployed.  The mechanism is fully compliant with both [RFC793] and

   Since the mechanism is an obfuscation technique, focus has been on a
   reasonable compromise between level of obfuscation and ease of
   implementation.  Thus the algorithm must be computationally
   efficient, and not require substantial data structures.

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2.  Randomising Ports

2.1  Ephemeral Port Range

   The Internet Assigned Numbers Authority (IANA) assigns the unique
   parameters and values used in protocols developed by the Internet
   Engineering Task Force (IETF), including well-known ports [IANA].
   IANA has traditionally reserved the following use of the 16-bit port
   range of TCP and UDP:

   o  The Well Known Ports, 0 through 1023.
   o  The Registered Ports, 1024 through 49151
   o  The Dynamic and/or Private Ports, 49152 through 65535

   The range for assigned ports managed by the IANA is 0-1023, the
   remainder is registered by IANA but not assigned.

   The ephemeral port range traditionally includes the 49152-65535
   range, and should also include the 1024-49151 range.  However, since
   this range include user specific server ports this may not always be
   possible.  A host should use the largest possible range, since this
   improves the obfuscation provided by randomising the ephemeral ports.

   Note that this method may also be used when dynamically reassigning
   ports as proposed by [Shepard].

2.2  Choosing the Port

   Choosing a random port can, if a suitable random source is available,
   be implemented as a simple random selection, i.e.:

       port = min_ephemeral + random() % (max_ephemeral - min_ephemeral)

                                Figure 1

   Several well-know operating systems use this approach.

   However, since the resulting connection four-tuple must be unique,
   the chosen port may already be in use with identical IP addresses and
   server port, thus the four-tuple is not unique.  Consequently
   multiple ports may have to be tried and verified against all existing
   connections before a port can be chosen.

   Although carefully chosen random sources and four-tuple lookup
   mechanisms optimised through e.g.  hashing, will mitigate the cost of
   this verification, some systems may still not like to incur this
   unknown search time.

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   Systems that are specially vulnerable to this kind of repeated
   four-tuple collisions are systems that create many connections from a
   single local IP address to a single service (i.e.  both IP addresses
   and peer port are fixed).  Gateways such as proxy servers are an
   example of such a system.

   Finding ports that result in a unique four-tuple are handled by some
   operating systems by having a global 'next ephemeral port' variable
   that is equal to the previously chosen ephemeral port + 1, i.e.  the
   selection process is:

       next_ephemeral_port = 1024;  /*initialisation, could be random*/

       do {
           port = next_ephemeral_port;
           if (next_ephemeral_port == max_ephemeral_port) {
               next_ephemeral_port = min_ephemeral_port;
           } else {
       } until (four-tuple is unique);

                                Figure 2

   We will refer to this as 'Algorithm 1'.  Note that the loop
   prevention mechanism has been left out for clarity.

   This works well, since the number of connections (globally, across
   all four-tuples) that has a life-time longer than it takes to exhaust
   the total ephemeral port range is small, thus four-tuple collisions
   are rare.

   However, this method has the drawback, that the 'next_ephemeral_port'
   variable and thus the ephemeral port range is shared between all
   connections and it is easy to predict the next ports chosen by the
   client.  If an attacker operates an innocent server to which the
   client connects, it is easy to obtain a reference point for the
   current value of 'next_ephemeral_port.

   Ideally, we would like a 'next_ephemeral_port' value for each set of
   (local/peer IP addresses, peer port).  These should be initialised
   with random values within the ephemeral port range and would thus
   separate the ephemeral port ranges of the connections entirely.
   Since we do not want to store all these 'next_ephemeral_port' values,
   we propose an offset function F(), that can be computed from the
   local/peer IP addresses, peer port and a secret key.  F() will yield
   (practically) different values for each set of arguments, i.e.:

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       next_ephemeral_port = 1024;  /*initialisation, could be random*/

       offset = F(local_IP, remote_IP, remote_port, secret_key);
       do {
           port = min_ephemeral +
                  (next_ephemeral_port + offset)
                      % (max_ephemeral - min_ephemeral);
       } until (four-tuple is unique);

                                Figure 3

   We will refer to this as 'Algorithm 2'.  Note that the loop
   prevention mechanism has been left out for clarity.

   In other words, the function F() provides a connection-local fixed
   offset of the global ephemeral port range controlled by
   'next_ephemeral_port'.  Both the 'offset' and 'next_ephemeral_port'
   variables may take any value within the storage type range since we
   are restricting the resulting port similar to that shown in Figure 1.
   This allows us to simply increment the 'next_ephemeral_port' variable
   and rely on the unsigned integer to simply wrap-around.

   The function F() should be a cryptographic hash function like MD5
   [RFC1321].  The function should use both IP addresses, the peer port
   and a secret key value to compute the offset.  The peer IP address is
   the primary separator and must be included in the offset calculation.
   The local IP address and peer port may in some cases be constant and
   not improve the connection separation, however, they should also be
   included in the offset calculation.

   Cryptographic algorithms stronger than e.g.  MD5 should not be
   necessary, given that port randomisation is a pure obfuscation
   technique.  The secret should be chosen as random as possible, see
   [RFC1750] for recommendations on choosing secrets.

   Note that on multiuser systems, the function F() could include user
   specific information, thereby providing protection not only on a host
   to host basis, but on user to service basis.

2.3  Secret Key

   Every complex manipulation (like MD5) is no more secure than the
   input values, and in the case of ephemeral ports, the secret key.  If
   an attacker is aware of which cryptographic hash function is being
   used by the victim (which we should expect), and the attacker can
   obtain enough material (e.g.  ephemeral ports chosen by the victim),
   the attacker may simply search the entire secret key space to find

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   To protect against this, the secret key should be of a reasonable
   length.  Key-lengths of 32-bits or 64-bits should be adequate, since
   a 32-bit secret would result in approximately 65k possible secrets if
   the attacker is able to obtain a single ephemeral port (assuming a
   good hash function).  If the attacker is able to obtain more
   ephemeral ports 64-bits or more should be used.

   Another possible mechanism of protecting the secret key is to change
   it after some time.  If the host platform is capable of producing
   reasonable good random data, the secret key can be changed.

   Changing the secret will cause abrupt shifts in the chosen ephemeral
   ports, and consequently collisions may occur.  Thus the change in
   secret key should be done with consideration and could be performed
   whenever one of the following events occur:

   o  Some predefined/random time has expired.
   o  The secret has been used N times (i.e.  we consider it insecure).
   o  There are few active connections (possibility of collision is
   o  There is little traffic (the performance overhead of collisions is
   o  There is enough random data available to change the secret key
      (pseudo-random changes should not be done).

2.4  Choosing Algorithm

   Algorithm 1 has the advantage, that it provides complete
   randomisation, but may not scale well with many simultaneous
   connections.  Algorithm 2 provides complete separation in local/peer
   IP address and peer port space, and only limited separation in other
   dimensions (See Section Section 2.3), however, this algorithm scales

   Thus Algorithm 1 should be used when the cost of choosing an
   ephemeral port is not important, or when the ratio of used ports and
   available ports are low (for given local/peer IP addresses and peer
   port).  A switch to algorithm 2 should happen if the cost of choosing
   an ephemeral port is important and when the ratio between used ports
   and available ports increase.

   Note that when the ratio between used ports and available ports
   increase, the obfuscation resulting from port randomisation decrease
   and has no effect when the entire port space is in use.

   The ratio where to switch between algorithms depend on the cost of

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   the four-tuple uniqueness test.  Systems capable of handling many
   simultaneous connections normally has an efficient PCB-lookup.
   However, verifying a four-tuple for uniqueness requires a lookup
   against all existing connections, even unconnected (but bound).
   Additionally, options exist, that will allow reuse of ports, making
   the detection even more complex than a PCB-lookup.  The the cost of a
   four-tuple verification may easily be many times that of a single PCB

   While the ratio is very implementation dependent and calculating the
   exact ratio may be difficult without using additional resources, an
   appropriate ratio can be estimated and used for an algorithm switch.
   E.g.  if the ephemeral port range contain N possible ports, the
   switch to algorithm 2 may happen when the total number of connections
   reach N/2.

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3.  Security Considerations

   Randomising ports is no replacement for cryptographic mechanisms,
   such as IPsec.

   An eavesdropper, which can monitor the ephemeral ports of other hosts
   (and thus also sequence numbers etc.) can easily hijack or corrupt
   the connection.  Randomising ports does not provide any additional
   protection against this kind of attacks.  In such situations stronger
   authentication techniques should be used.

   If the local offset function F() results in identical offsets for
   different inputs, the port-offset mechanism proposed in this document
   has no or reduced effect.

   If random numbers are used as the only source of the secret key, they
   must be chosen in accordance with the recommendations given in

   If all ports available in the ephemeral port range are in use,
   randomisation provides no obfuscation.

   If an attacker use dynamically assigned IP addresses, the current
   ephemeral port offset (Algorithm 2) for a given four-tuple can be
   sampled and subsequently be used to attack an innocent peer reusing
   this address.  However, this is only possible until a re-keying
   happens as described above.  Also, since ephemeral ports are only
   used on the client side (e.g.  the one initiating the connection),
   both the attacker and the new peer needs to be servers in the above
   scenario.  Although servers using dynamic IP addresses exist, they
   are not very common and with an appropriate re-keying mechanism the
   effect of this attack is limited.

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4.  Acknowledgements

   The offset function was inspired by the mechanism proposed for
   defending against TCP sequence number attacks [RFC1948].

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5.  References

5.1  Normative References

   [RFC793]   Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.

   [RFC768]   Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              April 1992.

   [RFC1750]  Eastlake, D., Crocker, S. and J. Schiller, "Randomness
              Recommendations for Security", RFC 1750, December 1994.

   [RFC1948]  Bellovin, S., "Defending Against Sequence Number Attacks",
              RFC 1948, May 1996.

   [RFC2385]  Heffernan, A., "Protection of BGP Sessions via the TCP MD5
              Signature Option", RFC 2385, August 1998.

5.2  Informative References

              Dalal, M., "Transmission Control Protocol security
              considerations", draft-ietf-tcpm-tcpsecure-01.txt (work in
              progress), June 2004.

   [Watson]   Watson, P., "Slipping in the Window: TCP Reset attacks",
              december 2003.

   [IANA]     "IANA Port Numbers",

   [Stevens]  Stevens, W., "Unix Network Programming, Volume 1:
              Networking APIs: Socket and XTI,  Prentice Hall", 1998.

   [Shepard]  Shepard, T., "Reassign Port Number option for TCP",
              draft-shepard-tcp-reassign-port-number-00 (work in
              progress), July 2004.

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Author's Address

   Michael Vittrup Larsen
   Skanderborgvej 232
   Aarhus  DK-8260

   Phone: +45 8938 5100
   EMail: michael.vittrup.larsen@ericsson.com

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